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9
Japanese Plum PollinationJAMES H. L RUE AND M xwELL V NORTON
Fruit of peach, plum, and nectarine trees do not
develop until after pollination and subsequent fer
tilization of the ovules in their flowers (fig. 9.1).
Pollen is produced in anthers and must be transferred to the top of the pistil (stigma) during pollina
tion . Pollen transfer (pollination) usually results from
insect activity (fig. 9.2), but may occur naturally; that
is, as a result of anther-stigma contact. Some move
ment of pollen from anthers mayoccur when branches
are buffeted by wind .
Pollen grains germinate on the stigma and pol
len tubes grow the length of the style to the ovary. A
sperm nucleus then travels within the pollen tube
from the pollen grain to the ovary where it fuses with
the egg nucleus (ovule) to form an embryo . In stone
fruits this fusion must occur before the fleshy, or
Style .n >I
Ovary ............. \
Ovule ll JACKET(CALYX)
Fig. 9.1. A plum flower showing reproductive parts.
edible portion of the fruit can develop. Without polli
nation, the flower will abort. Some varieties are self
fruitful - fruit will develop because pollination occurs
when pollen from the same flower or other flowerson the same tree result in development of an embryo;
thus, pollination from a "pollenizer" tree is not nec
essary. thers are self-unfruitful, requiring pollination
from another variety - cross-pollination to produce
fruit.
All commercial varieties of peaches and nectar
ines grown in California are self-fruitful. Plant breed
ers and growers alike consider self-unfruitfulness a
serious defect and discard any potential new variety
that does not pollenize itself.
European plums grown in California include
both self-frui tful and self-unfruitful varieties. On the
Fig. 9.2. Bees are responsible for transferring most poUen fromone plum Dower to anoth er.
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average , 30 percent of the flowers on self-fruitful
varieties will set whether the flowers have been cross
or self-pollinated. This is co nsidered more than
enough for a commercial set. Varieties of the self
unfruitful group have flowers that set only about 1 to
2 percen t, or less, insufficient fo r a commercial crop;
these must be interplanted with pollenizing varieties.
Table 9.1. Pollenizers for Japanese plums
Most commercial Japanese plum varieties grown
in California are either totally or partially self-unfruitful
and require cross-pollina tion to produce commercial
crops table 9.1).
Self-fruitful or partly se lf-frui tful varieties may
vary in degree of se lf-fruitfulness fro m rear to year
and from area to area. For example, 'Santa Rosa,'
Variety to bepollinated
Angeleno G G G
lackamber G G P G G F P F G G F F F
lack Beaut G F F F F F F F F F
Casselman"
Catalin a F F F F
EI Dorado F G P F G G F P P F G G
Friart F F F F F F F
Grand Rosa*
July Santa Rosa*
Kelsey F F F G G
aroda P F P G G P G G F F F F G F
Nubiana*
Queen Ann G P F G G P P P F G F
Queen Rosa G G G F P G G F
Red Beaut G G F P F F F F F F F F
Red Ro sa F
Red Roy F
Rosemary F F F F
Royal Diamond F
Royal Red G F F F F F F
Roysum
Santa Rosa"
Simka*
Spring Beaut G F F F F F F FWickson F I G P
Empress- P om stic (European). Use President or French Prune
President-P. domestic (European). Use Sugar, Standard, or French Prune
G=Good fruit set most years.F = Fair fruit set most years.P = Poor fruitor no fruit set most years.* =Self-fruitful , usually does not need a pollenizer,t =Fruitful, most years. Scattered pollenizers help some years.
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considered self-fruitful in the Fresno-Tulare county
area, usually produces well when planted in solid
blocks. However, in some northern California coun
ties, 'Santa Rosa' often requires cross-pollination by
bees from other varieties to enhance fruit set.
Two factors contributing to successful growth of
commercial Japanese plums in California are selec
tion of the proper pollenizer and distribution ofpollenizer trees or limbs throughout the orchard.
The wrong pollenizer or not enough pollenizers
within an orchard may limit production. Too many
pollenizer trees within an orchard often cause an
overset, resulting in high thinning costs, small fruit
size, or both.
Selecting PoUenizers
Not all plum varieties are good sources of pollen.
Some varieties have flowers that have limited quanti
ties or nonviable pollen. That is, they produce infertile pollen or pollen that germinates poorly. Some
flowers produce no pollen in the anthers. Pollen of
some varieties will germinate on the pistils of a few
other varieties, but not all. In some cases, the pollen
will germinate on the stigma, but the pollen tube will
not grow far enough or fast enough in the pistil to
reach the ovule while it is still viable.
To set satisfactory commercial crops on plum
varieties needing pollination, varieties must bloom at
the same time, be cross compatible, and provide
adequate pollen capable of setting fruit. A winter
providing adequate chilling often is followed by a
relatively short bloom period and good overlap ofpollenizer varieties. Favorable cross-pollination and
good fruit sets occur unless the bloom period is
extremely short with adverse weather. Following mild
winters, the bloom period is typically delayed and
protracted. Certain variety combinations may not
cross-pollinate effectively because their bloom peri
ods do not coincide (fig. 9.3) .Although self-unfruitful, some varieties may act
as pollenizers for other commercial varieties. Some
variety combinations planted in alternate rows may
only partially pollenize each other; thus, at least one
more pollenizer must be added to the orchard. Grow
ers often use "universal" pollenizers, such as 'Santa
Rosa' and 'Wickson,' that bloom over long periods
and produce large quantities of viable pollen. There
appears to be no exception to the rule that any
self-fertile plum 'will pollenize any other plum, assum
ing overlapping of bloom.
In planning a new Japanese plum orchard, pay
careful attention to the pollination requirements of
each variety table 9 .1) and orchard configuration
Fig. 9.3. Some plum variety combinations do not work as pollenizers because of different bloom times.
that w ll best satisfy these requirements . This can be
done by planting alternate rows of compatible varie
ties, scattering pollenizer trees at intervals through
out the orchard, or grafting pollenizer limbs into the
desired commercial variety.
PoUenizer rrangement
When arranging pollenizer trees in the orchard, the
most common scheme is to plant four inside rows of
one variety alternating with four inside rows of
another variety, each of which pollenizes the other.
Border rows have only two rows of one variety.
X X 0 0 0 0 X X X X 0 0
X X 0 0 0 0 X X X X 0 0
X X 0 0 0 0 X X X X 0 0
X X 0 0 0 0 X X X X 0 0
X X 0 0 0 0 X X X X 0 0
X X 0 0 0 0 X X X X 0 0
This system facilitates irrigation and harvest.
Where one variety is favored, an arrangement of four
inside rows of that variety alternating with two inside
rows of another pollenizer is commonly used.
0 0 X X 0 0 X X 0
0 0 X X 0 X X 0 0
0 0 X X 0 0 0 0 X X 0 0
0 0 X X 0 0 0 X X 0 0
0 0 X X 0 0 0 0 X X 0 0
0 X X 0 0 0 0 X X 0 0
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This arrangement usually assures good pollina
tion, while allowing greater production of the favored
variety. Easeof irrigation and harvest isstill maintained.
Where a minimum of pollenizer trees is required,
they are placed in the orchard in an orderly fashion
for picking and maintenance. Arrangements for sup
plying pollen to the main variety, while keeping the
number of pollenizers to a minimum, are common.
Examples include placing one tree of the pollenizer
to eight of the main variety, with each pollenizer
placed every third tree in every third row. In this
case, each tree of the main variety is adjacent to a
pollenizer tree.
0 0
X 0 0 X 0
0 0 0 0 0
0 0 0 0 0
X 0 0 X 0
0 0
This system is awkward because the pollenizer
trees scattered throughout the orchard must be
harvested separately. A somewhat easier way to dis
perse them is to plant them in a regular pattern
down the row, halfway between permanently set
trees. The pollenizer trees are planted 10 feet from
each tree in an orchard with 20-foot row spacings.
Each is pruned to a single upright t runk and allowed
to form a crown of branches about 8 to 10 feet high
(fig. 9.4). This confines the growth and space taken
by the pollenizer, but is usually adequate to supply
pollen to the rest of the orchard. Fruit in the small
pollenizer trees is knocked to the ground , as harvestis impractical.
FiR 9.4. PoUenizertrees pruned to an upright shape and grownin a regular pattern throughout the orchard.
PoUenizer imbs
Use of pollenizer limbs grafted directly into trees (fig.
9.5 is a common way of supplying pollen to plum
orchards in California. Besides supplying pollen
directly in the tree where needed , this system allows
the grower to plam solid blocks of one variety rather
than interplanring rows of one or more varieties for
pollination purposes.
Pollenizer limbs grafted directly into trees serve
two purposes. First , plum varieties that are difficult to
pollinate often have flowers with poor attraction for
bees. Bloom on the pollenizer limb serves to attract
bees to the tree where they visit flowers and collect
pollen. Second, a pollenizer limb located in a tree
serves as a base from which bees may "accidentally"
visit surrounding blooms that need to be pollinated.
Although they may not stay long because there is
little or no pollen or nectar, as in some varieties, the
briefvisit may last long enough for the pollen transfer
to take place.Pollenizer limbs are usually grafted into trees 1
year after planting. When the young trees are pruned
during the first dormant season, an extra limb is left
at a strategic location for grafting, usually on the
northwest corner of the young tree. After pruning,
the limb is whip grafted about 1 to 2 feet above the
tree's crotch. In spring, after the graft begins to grow,
interfering shoot growth is removed. Some shoot
growth on the same limb as the graft is allowed to
grow to the outside of the tree to help build a fruit
bearing scaffold limb around the graft. One or two
vigorous shoots from the graft should be encouraged
to grow straight up into the center of the tree. In thatposition they will serve well as a pollenizer but willnot interfere with the fruit-bearing area of the sur
rounding tree.
Most growers mark the pollenizer limb with
white latex paint so pruners will not accidentally
remove it. Fully developed pollenizer limbs must be
pruned to control growth and size. They may be
pruned in winter, but pruning is often delayed until
after bloom in spring to allow the largest number of
flowers to be present for pollination. Because of its
position high and in the center of the tree, it is
seldom commercially feasible to pick fruit from the
pollenizer. To prevent possible breakage, this fruitshould be knocked off with poles early in the season.
At least two, or even three, pollen izer varieties
should be grafted into trees in a pattern throughout
the orchard. This is to assure that at least one pollen
izer will bloom at the same time as the main variety
each year. f the plum variety is one that is easily
pollenized, only an occasional tree in a pattern
throughout the orchard may be grafted. f the variety
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Fig. 9.5. PoUenizer limbs grafted into the center of young plumtrees.
is difficult to pollenize, every tree in the orchard
should be grafted.
t is important to graft in the pollenizer limb
when the tree is 1 or 2 years old. Limbs grafted after
the tree is 3 to 4 years old are difficult to establish,
especially in the top of the tree where grafts tend to
have weak growth. A few plum varieties decline in
production after 15 to 20 years . At that time a pollen
izer grafted into a standard near the top of the tree,
although difficult to establish and maintain, improves
production.
Using Honeybees to Pollinate lums
Pollen of many stone fruit species is too heavy to be
carried by wind in sufficient quantities to produce a
good set. Thus, insects foraging in flowers for pollen
(their protein source) and nectar (their calorie
source) must be relied on to carry most of the pollen
(fig. 9.2) . As they forage, pollen grains cling to their
body hairs, and eventually are accidentally rubbed off
and become attached to the sticky surface of the
stigma where theygerminate
. Unknowingly, theseinsects carry pollen from one flower to the next and,
it is hoped, from one variety to the next.
Hundreds of species of wild insects forage blos
soms. None can be counted on to be always present
in sufficient numbers to transfer the amount of pol
len needed to assure a sufficient set; thus, honeybees
are used as pollinators.
Of the many kinds of bees, only the domestic
honeybee can be managed in a manner that will
facilitate moving large numbers of bees into an area
at any time. The alfalfa leafcutting bee and the alkali
bee are not adaptable to most fruit crops.
Some research has been conducted with so-called
orchard bees or the smia species, and mostly
with the Orchard Mason bee ( smia/ignatia in the
state of Washington. Mason bee culture involves build
ing special nest boxes to house the bees and placingthem in cold storage to coincide emergence time
with bloom time. This research is still in the experi
mental stage, and much remains to be learned about
managing these various wild bees.
ee placement n the orchard
When hives are placed in a new location, scout bees
immediately fly out in search of nectar and pollen
acceptable to the bees. Most bees forage on sources
close to the hive, but a few bees tend to fly much
further. Within 2 or 3 days it is normal to have some
bees collecting pollen or nectar from sources up to 4miles away from the hive. Once a good food source is
discovered, a bee will continue to work it until it
stops being attractive. Consequently, hives should be
brought into an orchard just as the first plum blos
soms are opening. Bees tend to work the first flowers
they find and, it is hoped , that is the orchard to be
pollinated.
Hives should be placed in groups of no more
than 6 to 12 per location. With more, bees become
too concentrated in one area; with less, beekeeping
operations become too inefficient.
In small blocks of less than 20 acres, hives can be
placed around the field with a higher concentration
near the center of long fields . Hives should be placed
along drives through the middle of larger blocks.
Some growers prefer to place hives on bin trailers
(fig . 9 .6) located in drive rows throughout the block.
Fig.9.6 . Beehivesplaced on bin trailers for placement through out the orchard.
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Hives should be placed in the warmest, driest
spot possible. n open, weed-free area, where the
early morning sun can warm the hives, is ideal. The
wanner the hives , the sooner the bees will begin
foraging. Tall grass should be avoided, as it keeps
hives damp and cool. Some growers place hives on
tar paper or black plastic that will absorb the morn
ingsun
and radiateheat into
them.Beehives are frequently stolen or vandalized or
are targets of passing motorists; thus, they should not
be placed near roads or where they can be easily
seen.
Eliminating competing blooms within or around
the plum orchard is important in maintaining bee
activity. Where possible , weeds and cover crops
should be mowed or disced under before bloom,
especially i weed species are present that are more
attractive to bees than the plum bloom.
Where bees are needed , one to two hives per
acre are usually sufficient. f there are too few pollen
izer trees, poor weather, weak hives, or very littlebloom overlap, two or more hives may be needed to
do the job adequately.
Not all plum varieties need the same amount of
cross-pollination each year to set commercial crops.
The number of pollenizer limbs or trees available is
most important in regulating the amount of fruit set.
Because weather conditions during bloom vary
each year, most growers prefer to place adequate
numbers of hives in their orchards to assure good
cross-pollination.
Normally, bees will not fly when air temperature
is less than 60°F l6°C) and at less than 55°F l30C
they seldom come out of the hive (fig. 9.7 ). Whenweather is dark and cloudy, bees tend to forage less.
Windy weather reduces flight activity and a I5-mph
wind will ground all honeybees. When there are only
a few hours of good bee weather each day, larger
numbers of foraging bees are necessary to obtain
adequate food for the bees and to provide adequate
pollination.
ive str ngth
The strength of beehives is measured by the number
of adult bees in the hive or the square inches of
brood immature stages) present. Hives used for pollination usually consist of two boxes (a hive body and
one super on top) ; each contains eight to ten rectan
gular frames where the bees build their combs.
Of prime importance to the grower is the num
ber of frames covered with bees. Agricultural com
missioners define a "frame of bees" as having three
fourths of the face of the frame covered on oth sides
with bees (fig. 9.8). The count must be adjusted to
Fig. 9.7. Bees seldom venture outside the hive or ly when temperatures are cold.
Fig. 9.8. Frame covered with bees.
Fig.9.9. Minimum strength hives should have six to eight framesof bees and an active queen .
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temperature conditions because when cold, bees
cluster tightly; when hot or when they are flying
heavily bees spread out.
A minimum strength hive for pollinating fruit
and nut trees should have at least six frames of bees
and a queen actively laying eggs. It is even more
desirable to have hives with eight frames (fig. 9.9) or
more, since more bees per hive means a greaternumber of them will be out foraging. An eight-frame
hive will have two or more times as many bees out
foraging as will a four-frame hive. Two four-frame
hives will not supply as many foragers as one eight
frame hive.
Most pollen col lected is used to feed the imma
ture brood. Hives with an actively laying queen will
raise high numbers of brood and will need large
supplies of pollen. Because pollen collectors are more
efficient pollinators than those foraging primarily for
nectar or water, it is important to have an active
queen and at least one frame of brood in each hive.
Growers can verify hive strength by asking the
beekeeper to open boxes and to pull out frames to
show how many are covered with bees and whether
the queen is actively laying eggs. Many growers hire a
third person, such as the county agricultural commis
sioner, to inspect a percentage of the hives and to
provide a certificate of strength. It is impossible to
just look at the entrance or under the lid and accu
rately determine the strength of the hive.
Hive inserts bouquetstraps attractants
A device inserted into the hive entrance and filled
with pollen from the desired pollenizer variety is
called a hive insert. When leaving the hive, bees
crawl through the insert and are dusted with pollen.
Theoretically, the bee then pollinates the first bloom
it lands on , regardless of what variety it flies to first.
This may be a temporary and partial solution in situa
tions where there simply are not enough pollenizers
or wrong pollenizers present. This technique does
not work well because many bees simply return to
the hive from the insert to unload their pollen.
Branches of pollenizer flowers are sometimes
cut from trees and placed in small containers of
water positioned in the trees to be pollenized. These
flowers last only a short time before wilting and may
need to be replaced two to three times. Again, this is
not a good method of supplying pollen and is only atemporary solution.
Pollen traps are used to remove pollen pellets
from bee legs before they can get back into the hive.
This reduces the amount of pollen being brought
into the hive so that more pollen foragers will have to
be sent out to compensate. Because pollen foragers
are more efficient than nectar foragers, the effective
ness of the hive as a source of pollinators should
be increased. While the proportion of pollen fora
gers is increased, fruit set increase has never been
demonstrated.
Many attempts have been made to spray water
soluble bee attractants on trees to be pollenized. Thescent or the taste of the attractant may attract bees
into the orchard, but once there, the bees must work
the flowers. The att ractant may only serve to confuse
the bees that end up licking any surface the material
is sprayed on - leaves, twigs, branches, weeds.
Another practice, baiting, consists of grinding up
flowers from the variety or crop to be pollenized and
putting them in the hive or steeping them in sugar
syrup. The hope is that bees will become tuned in to
the scent of the flowers and will be attracted to the
same flowers while foraging. Again, this approach has
not been commercially successful.
Additional Reading
Grout, R A, editor. 1973. The Hive and the Honey
bee. Dadant Sons, Hamilton, IlllaRue, ]. H., and M. H. Gerdts. 1976. Commercial
Plum Growing in California. University of California
Leaflet 458.
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